1. Field of the Invention
This invention relates to an improved device containing an inactive form of an acetylenic compound, having at least two conjugated C C groups, for measuring the integrated time-temperature history or integrated radiation-dosage history to which a perishable product has been exposed.
2. Brief Description of the Prior Art
Acetylenic compounds, containing at least two conjugated C.tbd.C groups, being cyclic or acyclic, symmetrical or unsymmetrical, diynes, triynes, tetraynes or hexaynes, and hereinafter referred to as diacetylenes, are an extremely useful class of compounds. They undergo 1,4-addition polymerization in the solid state, upon exposure to thermal annealing or actinic radiation, including ultraviolet and gamma radiation, thereby resulting in highly colored polymers. Utilization of this class of compounds has been described in U.S. Pat. No. 3,923,622 (Baughman and Yee to Allied Chemical, 1975) for use as cyclically bound ladder polymers; U.S. Pat. No. 3,999,946 (Patel, Preziosi and Baughman, to Allied Chemical, 1976) for use as time-temperature history indicators; U.S. Application Ser. No. 775,150, for use as crosslinking agents; U.S. Pat. No. 4,125,534 and a divisional application thereof being U.S. Application Ser. No. 929,062, for use as thermochromic indicating materials and in indicia-display devices; U.S. Application Ser. No. 772,190, now U.S. Pat. No. 4,125,534, as new carbazole-type photoconductors and non-linear optical materials; U.S. Application Ser. No. 839,542, now abandoned, for use in gradient-type recording devices, and U.S. Application Ser. No. 817,069, as new co-crystallized compositions.
As described in U.S. Pat. No. 3,999,946 and U.S. Application Ser. No. 839,542, diacetylenes can be used in indicators wherein exposure of a diacetylene compound, deposited on a substrate, to thermal annealing or actinic radiation will initiate a 1,4-addition polymerization resulting in a color change. The color change product at any given point in time represents an integrated time-temperature history of thermal annealing or integrated exposure to actinic radiation.
The reference, U.S. Pat. No. 3,999,946, (see the sentence bridging columns 7 and 8; column 6, lines 16-19; column 7, lines 50-51) generally describes the preparation of the indicators by coating crystals of an active diacetylene compound onto a substrate which is then responsive to 1,4-addition polymerization. However, the prepared indicators have the disadvantage that they must be stored at low temperatures and protected from actinic radiation prior to use since the 1,4-addition polymerization process is initiated by thermal annealing or exposure to actinic radiation. The step of protecting the formed indicators, at low temperature during storage and shipping to the point of use, greatly increases the cost of production and introduces an element of uncertainty as to the reliability of the final readings of the devices after use.
What is needed is a device containing a form of diacetylene which can be stored at ambient temperatures during and after manufacture, shipped by standard procedures, and which can be applied to an article and activated by simple means just prior to use.
The above reference, U.S. Pat. No. 3,999,946, generally describes the use of active phases of diacetylenes which undergo an irreversible color change upon exposure to thermal annealing or actinic radiation. Techniques are also described for deactivating the active phase of a diacetylene after use, by adding an inhibitor, changing the temperature, or by applying a mechanical stress in order to decrease the thermal reactivity and permanently record the integrated time-temperature history obtained. It is disclosed that either an active or an inactive form of a given compound may be crystallized from a solvent, depending on the concentration (see column 9, lines 16-28). However, the reference does not describe or suggest the use of the inactive forms of diacetylenes in forming the indicator for use as an integrated time-temperature history indicator.
We have found that inactive and active forms of diacetylenes exhibit different X-ray diffraction patterns and the inactive form is incapable of undergoing 1,4-addition polymerization upon exposure to gamma radiation at room temperature or thermal annealing below its melting point.
Furthermore, we have found that inactive forms of diacetylenes can be coated on a substrate and converted to an active form prior to use by solvent or melt recrystallization processes. Thus, problems associated in storing and shipping the indicator prior to use, are eliminated.